1. Field of the Invention
The present invention relates to a method of driving an optical wavelength converter module comprising a semiconductor laser which emits a laser beam as a fundamental wave and an optical wavelength converter device in the form of an optical waveguide which converts the wavelength of the laser beam.
2. Description of the Prior Art
Various attempts have heretofore been made to convert the fundamental of a laser beam into its second harmonic, e.g., to shorten the wavelength of a laser beam, using nonlinear optical material. One well known example of an optical wavelength converter device for effecting such laser wavelength conversion is a bulk-crystal-type converter device as disclosed, for example, in Introduction to Optical Electronics, pages 200-204, written by A. Yariv and translated by Kunio Tada and Takeshi Kamiya (published by Maruzen K.K.). This optical wavelength converter device relies upon the birefringence of a crystal in order to meet phase matching conditions. Therefore, any material which does not exhibit birefringence or exhibits only small birefringence, even if it has high nonlinearity, cannot be employed.
To solve the above problem, there has been proposed a fiber-type optical wavelength converter device. An optical wavelength converter device of this type is in the form of an optical fiber comprising a core made of a nonlinear optical material and surrounded by a cladding. One example of such an optical fiber is shown in vol. 3, No. 2, pages 28-32, of the Bulletin of the Microoptics Research Group of a gathering of the Japanese Applied Physics Society. Recently, much effort has been directed to the study of a fiber-type optical wavelength converter device since it can achieve gain phase matching between a fundamental and its second harmonic.
There are also known optical wavelength converter devices comprising a two-dimensional optical waveguide which is made of a nonlinear optical material and sandwiched between two substrates that serve as a cladding, as disclosed in U.S. Pat. No. 4,820,011, for example. Another known optical waveguide converter device comprises a three-dimensional optical waveguide which is made of a nonlinear optical material and embedded in a substrate of glass. The three-dimensional optical waveguide emits second harmonics into the substrate. These known optical-waveguide-type optical converter devices also offer the same advantage as that of the fiber-type optical waveguide converter device.
U.S. patent application Ser. No. 328,266 discloses in detail the generation of the sum of and the difference between the frequencies of two fundamentals with a fiber-type optical waveguide converter device. The generation of such sum and differential frequencies is also disclosed in detail in U.S. patent application Ser. No. 328,266. It is also known in the art that third harmonics can be generated using a nonlinear optical material having nonlinearity of the third order.
The waveguide of each of fiber- and optical-waveguide-type optical wavelength converter device is made of a nonlinear optical material. However, only the cladding or both the waveguide and the cladding may be made of a nonlinear optical material. Since part of a guided wave which is propagating through the waveguide enters as an evanescent wave into the cladding, the wavelength of the evanescent wave can be converted if the cladding is made of a nonlinear optical material.
An optical-waveguide-type optical wavelength converter device is often combined with a semiconductor laser which emits a laser beam as a fundamental, and the combination is used as an optical waveguide converter module. The efficiency with which a wavelength is converted by an optical wavelength converter device is proportional to the square of the intensity of the applied fundamental (more precisely, the intensity of the fundamental which travels through a nonlinear optical material) which is used for the generation of a second harmonic, to the cube of the intensity of the fundamental which is used for the generation of a third harmonic, and to the product of the intensities of two fundamentals which are used for the generation of sum and differential frequencies. In order for the optical wavelength converter module to produce a wavelength-converted wave of high intensity, therefore, it is highly effective to increase the output power of the semiconductor laser.
One known method of increasing the output power of a semiconductor laser is to drive the semiconductor laser with pulses. When the semiconductor laser is driven with pulses and the duration of each pulse is shorter than a certain time, the semiconductor laser is less liable to become thermally saturated, and can produce a peak output power higher than when it is continuously driven by a continuous drive signal.
If the semiconductor laser of the above optical wavelength converter module is driven with a pulsed drive current, then it is theoretically expected that the optical wavelength converter module will generate a wavelength-converted wave of higher intensity than when the semiconductor laser is continuously driven. When the semiconductor laser is actually driven with a pulsed drive current, the wavelength conversion efficiency of the optical wavelength converter module does become higher in certain instances than when it is continuously driven, but it also becomes lower in other instances.